The Death of the Dinosaurs: UNO Department of Geology's Contribution

by
Dale H. Easley1Department of Natural and Applied Sciences
University of Dubuque

8/31/2000

A widely accepted hypothesis about the cause of the extinction of the dinosaurs
at the end of the Cretaceous Period is an impact of a meteorite or comet (bolide)
in the Yucatan Peninsula, Mexico. The bolide is believed to have been about 6
miles in diameter, impacting the Yucatan at a low angle, vaporizing rock,
setting off catastrophic fires and tidal waves, and blanketing the Earth with
dust, which led to acid rain and a plunge in global temperatures. The angle was
such that central and western North America were particularly ravaged. As the
dust settled out of the atmosphere, large amounts of water vapor remained,
causing a rapid rise in temperature to follow the cooling. Thus the dinosaurs
and other Cretaceous life were hit with a one-two punch that caused massive
annihilation. The acceptance of this hypothesis is relatively new---the
previous edition of our historical geology book doesn't even mention it.
Evidence stored at the UNO Geology Department helped provide the support that
led to acceptance.

Former professor Al Weidie has had a Mexican connection dating back to his
graduate school days working on Gulf Coast geology. (As part of his student
accomplishments, he brought home a lovely wife, Ana, from Saltillo, Mexico.)
After coming to UNO and helping start the Department of Earth Sciences, Dr. Weidie
convinced PEMEX, the Mexican petroleum company, to ship him some cores from
wells that PEMEX drilled in the Yucatan Peninsula. (Cores are samples of the
rock being drilled through.) When Dr. Bill Ward joined the faculty in 1970,
the cores were available for a thesis project by his first graduate student, Bob
Marshall.

Cores from near the Cretaceous-Paleogene boundary proved to be difficult to
understand. A breccia there was particularly problematic, causing some to
accuse Dr. Ward of looking at concrete that had been drilled out in the
wells. However, in the breccia were fragments of anhydrite, limestone, and
dolomite, and the matrix was dolomitized. Dr. Ward reasoned that all the
anhydrite chunks and splinters in the breccia, which spread across the peninsula,
would amount to a lot of anhydrite. Its likely source was the Middle Cretaceous,
deeper underground. How could it have gotten to its present location?
Anhydrite is very soluble, so it had to have been moved quickly and without a
lot of water. The mechanism Dr. Ward hypothesized at the time was block
faulting. Years later, electric logs and seismic would show that this
explanation couldn't be supported.

In the years after Marshall's thesis, a high concentration of iridium at the
Cretaceous-Paleogene boundary was found in many places around the globe. The
likely source of iridium is extraterrestrial, in comets or meteorites. Its wide
distribution at a single time indicated the possibility of a massive impact.
The timing also corresponded with the end of the Age of Reptiles. The hunt was
on for a large crater caused by the impact. Geophysical evidence indicated the
presence of such a crater in the subsurface of the Yucatan Peninsula.

Geologists seldom believe geophysicists unless there is supporting evidence from
rock data. The PEMEX wells were known to exist, including one near the center
of the proposed impact crater, but the cores from them were supposedly lost.
However, someone came across a reference to Marshall's thesis and contacted UNO.
In the cores, they found shocked crystals of quartz that have been impacted
violently like hitting them with a hammer. Other evidence soon followed,
including finding glass microspherules throughout the Caribbean and as far away
as Wyoming. In 1995, Dr. Ward and others authored an article that appeared
in Geology, presenting much of the petrographic evidence from the
subsurface sedimentary rocks for the bolide impact, including a thin section of
anhydrite fragments wrapped in altered glass.

As part of a long series of programs on dinosaurs narrated by Walter Cronkite,
there was a segment on dinosaur extinction and its possible relation to the
Yucatan impact. A camera crew came to UNO and filmed a geologist going into the
lab where the cores were stored, across the hall from Dr. Ward's office.
The geologist had a flashlight in his hand, looking like an archaeologist
entering King Tut's tomb. Suddenly he discovered the long-missing cores. Of
course, the geologist passed up a light switch at the door. It looked as if UNO
didn't pay the electric bill. Yet there is no doubt that the missing cores
helped shed light on the cause of the extinction of the dinosaurs.

Footnotes:

1Many thanks to Dr. Bill
Ward for his recounting to me the history of UNO's involvement in studying the
Yucatan Impact Crater.